The present invention relates to a process for the preparation of a compound of the general formula I 
where R1, R2, R3 are hydrogen, C1- to C20-alkyl, C2- to C20-alkenyl, C2- to C20-alkynyl, C3- to C12-cycloalkyl, C4- to C20-cycloalkyl-alkyl, C1- to C20-hydroxyalkyl, or aryl or C7- to C20-arylalkyl which is unsubstituted or substituted by C1- to C8-alkyl, C1- to C8-alkoxy, halogen, C1- to C4-haloalkyl, C1- to C4-haloalkoxy, phenyl, phenoxy, halophenyl, halophenoxy, carboxyl, C2- to C8-alkoxycarbonyl or cyano, or R1 and R2 or R3 together are a C2- to C9-alkanediyl unit which is unsubstituted, monosubstituted or disubstituted by C1- to C8-alkyl, C1- to C8-alkoxy and/or halogen and in which one or two methyl groups may also be replaced by a (CHxe2x95x90CH) unit and R3 is additionally an acetylated carbonyl group in which the alkoxy groups are derived from an alcohol of the general formula II
R4xe2x80x94OHxe2x80x83xe2x80x83II
where R4 is C1- to C6-alkyl, and
U is an acetylated carbonyl group in which the alkoxy groups are derived from an alcohol of the general formula II, or is a compound of the general formula III
R3xe2x80x94Vxe2x80x94Wxe2x80x94R1xe2x80x83xe2x80x83III
where R1 is as defined under the formula I, and R3 is exclusively aryl which is unsubstituted or substituted by C1- to C8-alkyl, C1- to C8-alkoxy, halogen, C1- to C4-haloalkyl, C1- to C4-haloalkoxy, phenyl, phenoxy, halophenyl, halophenoxy, carboxyl, C2- to C8-alkoxycarbonyl or cyano,
V is a carbonyl group or is as defined for U under the formula I, and
W is as defined for V, with the proviso that one of the groups V and W is a carbonyl group and the other is an acetylated carbonyl group,
or
a compound of the general formula IV
R3xe2x80x94Vxe2x80x94Wxe2x80x94Oxe2x80x94R4xe2x80x83xe2x80x83IV
where R4 is as defined under the formula II, V and W are as defined under the formula III and R3 is as defined under the formula III,
by subjecting a compound of the general formula V 
where V, R1, R2 and R3 are as defined under the formula I or III, with the proviso that
in the case where a compound of the formula III is desired, use is only made of a compound Va in which
R1 is exclusively hydrogen and
R3 is exclusively aryl which is unsubstituted or substituted by C1- to C8-alkyl, C1- to C8-alkoxy, halogen, C1- to C4-haloalkyl, C1- to C4-haloalkoxy, phenyl, phenoxy, halophenyl, halophenoxy, carboxyl, C2- to C8-alkoxycarbonyl or cyano, and
in the case where a compound of the formula IV is desired, use is only made of a compound Vb in which
R1 and R2 are exclusively hydrogen,
R3 is exclusively aryl which is unsubstituted or substituted by C1- to C8-alkyl, C1- to C8-alkoxy, halogen, C1- to C4-haloalkyl, C1- to C4-haloalkoxy, phenyl, phenoxy, halophenyl, halophenoxy, carboxyl, C2- to C8-alkoxycarbonyl or cyano,
To an electrochemical reaction with an alcohol of the general formula II in the presence of an auxiliary electrolyte and catalytic amount of a metal salt (S) derived from a metal from the 1st, 2nd, 6th or 8th subgroup (subgroups IB, IIB, VIB or VIIIB; also known as Groups 11, 12, 6 and 8-10, respectively) of the periodic table, or from lead, tin or rhenium.
EP-A-460 451 discloses a process for the preparation of xcex1-hydroxymethyl ketals by electrochemical oxidation of aldehydes or ketones in the presence of alcohols and halogen compounds as auxiliary electrolytes. Repetition of the examples shows that more highly oxidized carbonyl compounds are also formed under the process conditions described if the carbonyl group is in the xcex1-position to an aromatic radical. Thus, for example, a methylene group in the xcex1-position to the carbonyl group can be oxidized to the carbonyl function and in addition the aldehyde or keto carbonyl group originally present can be oxidized to the carboxyl group. Thus, it is not only xcex1-hydroxyketals that are formed, but also xcex1-ketaldehydes, xcex1-ketoacetals, xcex1-ketalcarboxylic esters and xcex1-keto orthoesters. However, this process is still not entirely satisfactory since the overall yield of these target products is relatively low and in addition large amounts of other substantially unusable products are formed.
German Patent Application 19904929, which is not a prior publication, relates to a process for the preparation of 2,2,3,3-tetramethoxypropanol by electrochemical oxidation of methylglyoxal dimethyl acetal using a mixture comprising methanol, water and an auxiliary electrolyte as electrolysis medium and an iron, steel, platinum or zinc cathode.
It is an object of the present invention to provide an electrochemical process by means of which xcex1-hydroxyketals, xcex1-ketalaldehydes, xcex1-ketoacetals, xcex1-ketalcarboxylic esters and xcex1-keto orthoesters can be prepared from keto or aldehyde carbonyl compounds. We have found that this object is achieved by the. process defined above.
The process according to the invention is particularly suitable for the preparation of compounds of the general formulae I, III and IV, where the radical R4 in the acetylated carbonyl group is derived from methanol or ethanol.
Of the compounds of the formula I, preference is given to those of the formula Ia 
where U is as defined in formula I,
n is 0, 1, 2 or 3, and
R5 is C1- to C8-alkyl, C1- to C8-alkoxy, halogen, C1- to C4-haloalkyl, C1- to C4-haloalkoxy, phenyl, phenoxy, halophenyl, halophenoxy, carboxyl, C2- to C8-alkoxycarbonyl or cyano.
Preference is likewise given to compounds of the general formula IIIa 
where n, V, W and R5 are as defined under the formula Ia or III,
or of the general formula IVa 
where n, V, W, R4 and R5 are as defined under the formula Ia or IIIa.
These compounds are prepared by employing as starting compound of the general formula V a compound of the general formula Va 
where n and R5 are as defined under the formula Ia.
The process is furthermore particularly suitable for the preparation of compounds of the general formula:
H2m+1Cmxe2x80x94CHOHxe2x80x94CH(OR4)2
where m is a number from 1 to 10, and R4 is as defined in formula II, and for whose preparation use is made of a compound of the general formula:
H2m+1Cmxe2x80x94CH2xe2x80x94CHOxe2x80x94
The process is very particularly suitable for the preparation of
2-phenyl-2,2-dimethoxyethanol, 2-phenyl-2,2-dimethoxyacetaldehyde and 2-phenylglyoxal dimethyl acetal from methanol and acetophenone
xcex1-hydroxyoctanal dimethyl acetal from octanal and
2,2,3,3-tetramethoxypropanol from methylglyoxal dimethyl acetal.
The auxiliary electrolyte present in the electrolysis solution is generally a halogen-containing auxiliary electrolyte, such as elemental halogen, an alkyl halide or a hydrogen halide. Halogen-containing salts, in particular iodides or bromides, can also preferably be employed. Examples are ammonium halides, such as ammonium bromide, ammonium iodide and tetrabutylammonium iodide. Particularly preferred metal halides are furthermore alkali metal halides, such as sodium bromide, sodium iodide, potassium iodide and potassium bromide.
The metal salts (S) are preferably those derived from mineral acids. The anions of the metal salt are thus, for example, phosphate, sulfate, nitrate, perchlorate or halide.
The cations of the metal salt (S) are preferably iron, nickel, platinum, palladium, cobalt, zinc, silver or copper ions. The metal salt (S) is generally added to the electrolysis solution in amounts such that its metal ions are present therein in amounts of from 1 to 1000 ppm by weight, preferably from 5 to 500 ppm by weight, particularly preferably from 5 to 300 ppm by weight, based on the total amount of electrolysis liquid.
If desired, conventional co-solvents are added to the electrolysis liquid. These are the inert solvents having a high oxidation potential which are generally conventional in organic chemistry. Examples which may be mentioned are dimethyl carbonate and propylene carbonate. Besides said co-solvents, water can also be added to the electrolysis liquid, although the water content should not exceed 5% by weight, based on the total amount of electrolysis liquid.
In general, the electrolysis liquid has the following composition:
a starting compound of the general formula V
an alcohol of the general formula II
a halogen-containing auxiliary electrolyte
catalytic amounts of the metal salt (S)
possibly the desired products of the general formulae I, III and IV
possibly other by-products of electrolysis which are derived from the compounds of the general formulae I, II, III, IV and V
if desired, other conventional co-solvents.
The ratio between the products of the general formulae I and V and the other by-products to the starting compounds in the electrolysis liquid and the ratio of the individual products having different degrees of oxidation to one another is of course, dependent on the progress of the reaction.
The ratio between the products of the general formulae I, III, IV and V and the other by-products to the starting compounds in the electrolysis liquid and the ratio of the individual products having different degrees of oxidation to one another is of course dependent on the progress of the reaction.
In general, the amount of charge expended for the reaction is from 1 to 7 F per mole of starting compound of the general formula V. From 3.5 to 4 F are preferably employed if mixtures are desired which are intended to contain, as principal components, compounds of the formulae I and III, and from 4.5 to 5.5 F are employed if mixtures are desired which are intended to contain, as principal components, compounds of the formulae I and IV.
The process according to the invention can be carried out in all conventional types of electrolysis cell. Preference is given to undivided flow cells.
The current densities at which the process is carried out are generally from 0.5 to 25 A/dm2. The temperatures are usually from xe2x88x9220 to 60xc2x0 C., preferably from 0 to 60xc2x0 C. The process is generally carried out at atmospheric pressure. Higher pressures are preferably used if higher temperatures are to be used in order to prevent the starting compounds or co-solvents from boiling.
Examples of suitable anode materials are noble metals such as platinum, or metal oxides, such as ruthenium or chromium oxide, or mixtures of the RuOx/TiOx type. Preference is given graphite or carbon electrodes.
Suitable cathode materials are generally iron, steel, nickel, and noble metals, such as platinum and graphite and carbon materials.
When the reaction is complete, the electrolysis liquid is worked up by general separation methods. To this end, the electrolysis liquid is generally first distilled, and the individual compounds are obtained separately in the form of different fractions. Further purification can be carried out, for example, by crystallization or chromatography.
All experiments were carried out in an undivided cell having 11 bipolar electrodes (10 gaps, gap separation 1.5 mm).
Current density: 3.4 A/dm2 
Flow rate: 400 l/h